The present invention relates to a continuous process for the preparation of nitrobenzene by the adiabatic nitration of benzene with a mixture of sulfuric and nitric acids (so-called ‘mixed acid’). Such a process was first claimed in U.S. Pat. No. 2,256,999 and is described in more modern embodiments in U.S. Pat. No. 4,091,042, U.S. Pat. No. 5,313,009 and U.S. Pat. No. 5,763,697.
A common feature of the adiabatic processes described is that the starting materials, benzene and nitric acid, are reacted in a large excess of sulfuric acid, which absorbs the heat of reaction evolved and the water formed in the reaction.
The reaction procedure generally involves combining the nitric acid and sulfuric acid to give so-called ‘nitrating acid’ (also called ‘mixed acid’). Benzene is metered into this nitrating acid. The reaction products are essentially water and nitrobenzene. In the nitration reaction, benzene is used in at least the stoichiometric amount, but preferably in 2% to 10% excess, based on the molar amount of nitric acid. According to the state of the art, the crude nitrobenzene formed in the reaction apparatuses and separated from the acid phase in the phase separation apparatus is washed and worked up by distillation, as described e.g. in EP 1 816 117 A1 (page 2, lines 26 to 42), U.S. Pat. No. 4,091,042 (cf. above) or U.S. Pat. No. 5,763,697 (cf. above). A characteristic feature of this work-up is that, after washing, unreacted excess benzene is separated from nitrobenzene in a final distillation and re-used in the nitration reaction as recycle benzene, which also comprises low-boiling non-aromatic organic compounds (so-called ‘low boilers’) (cf. DE 10 2009 005 324 A1). The treatment of the exhaust gas from the adiabatic nitration reaction is described in EP 0 976 718 B1. The exhaust gas from the acid circuit and final crude nitrobenzene is drawn off, combined and passed through an NOx absorber to recover dilute nitric acid, which is returned to the reaction. The sulfuric acid, referred to as circulating acid, is concentrated in a flash evaporator and freed of organics as far as possible. High-boiling organics, e.g. nitrobenzene, dinitrobenzene and nitrophenols, and traces of benzene, remain in the circulating acid and are therefore also returned to the reaction.
When the exhaust gas from an adiabatic nitration reaction is worked up as described in EP 0 976 718 B1, i.e. when the exhaust gas from the acid circuit and final crude nitrobenzene is drawn off, combined and passed through an NOx absorber to recover dilute nitric acid, which is returned to the reaction, it is advisable not to return this dilute nitric acid to the reaction until the start-up process (meaning the period of time within which a production plant is brought to target load from shutdown; cf. below for details) has ended, because admixing of the dilute nitric acid reduces the overall concentration of the starting nitric acid and causes reaction kinetics to slow down.
The quality of an adiabatic process for the nitration of aromatic hydrocarbons is defined on the one hand by the content of unwanted reaction by-products in the product, which are formed by multiple nitration or oxidation of the aromatic hydrocarbon or the nitroaromatic. In the preparation of nitrobenzene one strives to minimize the content of dinitrobenzene and nitrophenols, particularly trinitrophenol (picric acid), which is classified as explosive. The quality of an adiabatic process is defined on the other hand by the fact that the process can be operated without a technical shutdown of production.
To obtain nitrobenzene with particularly high selectivities, the nature of the mixed acid to be used has been stipulated in detail (EP 0 373 966 B1, EP 0 436 443 B1 and EP 0 771 783 B1) and it has been pointed out that the content of by-products is determined by how high the maximum temperature is (EP 0 436 443 B1, column 15, lines 22 to 25). It is also known that a high initial conversion is advantageous for a high selectivity and that this is achieved if optimum mixing is applied at the beginning of the reaction (EP 0 771 783 B1, paragraph[0014]).
Outstanding selectivities are obtained when the chosen initial reaction temperature is very low (WO 2010/051616 A1), although this is tantamount to increasing the reaction time several fold. A high space-time yield is advantageous for the industrial application of a process, since this makes it possible to construct compact reaction equipment distinguished by low capital expenditure in relation to capacity. This approach is therefore counter-productive.
Common to all the literature references listed is the fact that they do not describe the start-up process of a nitration unit and its difficulties.
As regards the quality of the auxiliary material sulfuric acid in the adiabatic preparation of nitrobenzene, EP 2 070 907 A1 describes that low contents of metal ions in the sulfuric acid obtained from the nitration have a positive effect on the concentration of the sulfuric acid. Thus, in the flash evaporation (i.e. evaporation associated with expansion) of the waste acid comprising sulfuric acid, which is obtained after separation of the aqueous phase from the reaction mixture obtained from the nitration of benzene, higher sulfuric acid concentrations are achieved in the resulting concentrated sulfuric acid when the content of metal ions is low. This is probably attributable to the improved evaporability of the water in the flash evaporator when there are low contents of metal ions in the waste acid. It has thus been found that, in flash evaporation under otherwise identical conditions (same temperature of the waste acid, same sulfuric acid content of the waste acid, same pressure in the flash evaporator), the concentration of H2SO4 in the concentrated sulfuric acid obtained is up to 0.25% higher when using a waste acid with low contents of metal ions of less than 900 mg/l.
EP 2 070 907 A1 further describes that lower metal ion concentrations in the sulfuric acid also result in a lowering of the boiling point of the sulfuric acid, which in turn reduces the amount of energy required to concentrate the sulfuric acid.
EP 2 070 907 A1 further points out that the problematic deposits of metal sulfates can be found not only in heat exchangers but also at any points where the concentration of the metal ions that form hardly soluble metal sulfates is sufficiently high and the temperature sufficiently low to cause the formation of solids, and where at the same time the flow rate of the sulfuric acid or the cross-section of the sulfuric acid pipelines is sufficiently small to cause an accumulation of the metal sulfates that interferes with the process. Therefore, not only can metal sulfate deposits be observed in heat exchangers, but metal sulfates can also occur as deposits on the bottom of tanks, at measuring points like level measurings, and on dispersing elements, which conventionally have small flow orifices. Likewise, metal sulfate deposits can also occur inside the flash evaporators, where the sulfuric acid is conventionally cooled while water is evaporated and the concentration of the acid is increased. Moreover, deposits of metal salts can also form in the work-up steps following the reaction, e.g. in the effluent work-up, due to entrained metal sulfates. According to the state of the art cited above, necessary provision is made for a periodic cleaning of the affected parts of the plant in order to reduce the interference caused by these deposits. However, this cleaning entails production shutdowns and hence additional costs. The cleaning of heat exchangers and pipelines that convey sulfuric acid to remove solid precipitated metal sulfates can be dispensed with if, in the nitration of the benzene by a mixed acid comprising sulfuric and nitric acids, the sulfuric acid recovered by the flash evaporation of water is not completely recycled into the reaction zone as recycled acid, but partially removed and replaced by fresh sulfuric acid which is poor in metal ions.
EP 2 070 907 A1 does not go into organic compounds which can concentrate in the circulating sulfuric acid, above all when the production plant is operated at high loads.
It is true that the processes of the prior art described succeed in preparing a nitrobenzene having a low content of by-products, i.e. comprising only from about 100 ppm to 300 ppm of dinitrobenzene and 1500 ppm to 2500 ppm of nitrophenols, wherein picric acid can make up a proportion of 10 mass percent to 50 mass percent of the nitrophenols. The processes are also distinguished by a high space-time yield. However, only processes that are already in progress are described, i.e. processes in which the period from the beginning of the reaction until the target load is achieved (so-called ‘start-up period’) has already passed. Any particular difficulties during the start-up of an adiabatic nitration process are not mentioned.